The present invention relates to an improved apparatus and method for measuring the intratracheal pressure of an intubated patient, and particularly, to such an apparatus and method suitable for use when an endotracheal tube is placed in the patient for a long period of time.
Patients undergoing post-operative or intensive care treatment are often mechanically ventilated through an endotracheal tube. An endotracheal or intubation tube is a flexible tube, typically formed of plastic. The tube is inserted through the mouth so that the distal end of the tube is located in the trachea and the proximal end extends out of the mouth of the patient. The distal end of the tube is retained in the trachea by an inflatable cuff surrounding the tube. The cuff can be filled with air by means of a syringe connected to an inflation passage at the proximal end of the tube.
A typical mechanical ventilator employed with an endotracheal tube has a breathing circuit comprising an inhalation limb and an exhalation limb connected to two arms of a Y-connector. The third arm of the Y-connector is connected, via a patient limb, to the proximal end of the endotracheal tube. The ventilator supplies breathing gases to the patient though the inhalation limb during inhalation. The contraction of the subject's lungs discharges breathing gases through the exhalation limb during exhalation. In addition to supplying breathing gases, the ventilator may be used to supply pharmaceutical agents for ventilation therapy by entraining the agents in the inhaled breathing gases.
Conditions in the breathing gas supply equipment and in the respiratory system of the patient are often monitored by one or more sensors connected along the flow path of the breathing gases to and/or from the patient. Parameters measured may include one or more of the concentration of a particular respiratory gas or gases, airway pressure, and airway volumetric gas flow. The pressure and flow characteristics or waveforms are particularly useful in deriving information regarding the lung mechanics of the patient. They may also be utilized to ascertain the presence of leaks and/or occlusions along breathing gas paths.
Prevailing medical opinion is currently of the view that, in the past, there has been a tendency to ventilate diseased lungs too aggressively in efforts to cure or alleviate the effects of lung disease. New guidelines have recently been introduced to provide direction in ventilating lungs in a manner now deemed preferable. These guidelines emphasize the importance of lung monitoring. Accurate parameter measurement is a vital aspect of effective lung monitoring.
The major variables of lung mechanics are the resistance to the passage of breathing gases and the compliance, or elasticity, of the patient's respiratory organs, primarily the lungs. The values of these parameters change during the course of serious lung disease and decisions to administer ventilation therapy, and the patient's response thereto, can be determined from the measured values.
In intubated patients, the resistance to the passage of breathing gases at the output of the ventilator comprises the resistance of the tracheobronchial tree of the patient in series with the resistance of the breathing gas pathway, including the endotracheal tube. This resistance is often called the total airway resistance. Since the flow through the series connected resistances is the same, when measuring gas flows, the flow sensor may be located proximally, distally, or at any desired location along the breathing gas flow path. However, the same is not true with respect to the measurement of gas pressures. In the measurement of gas pressures, it is important to measure the pressure as close to the alveoli of the lungs as possible. This is because the pressure drop along the endotracheal tube, caused by the respiratory gas flow, can be considerably greater in magnitude than the alveolar pressure. Thus, the standard practice of monitoring airway pressure at the proximal end of the endotracheal tube often gives misleading measurement data concerning the pressure conditions actually existing within the lungs of the patient. For example, a peak pressure of 40 cmH.sub.2 O may be measured at the proximal end of the endotracheal tube when, in fact, only 20 cm H.sub.2 O pressure exists in the lungs of the patient. This causes difficulties in calculating the resistance of the system and compliance of the lungs.
Also, the exhalation time of the patient may be too short to allow the lungs to empty completely of breathing gases. The pressure remaining in the patient's lungs, which may be characterized as an intrinsic positive end expiratory pressure, or PEEP, may remain undetected under current measurement techniques, due to the considerably higher pressure drop along the endotracheal tube. Either of these circumstances may lead to inaccurate evaluation of the condition or progress of the patient and to inappropriate therapeutic intervention.
For the foregoing, and other, reasons, it would be much more desirable to continuously monitor airway pressure at the distal end of the endotracheal tube so that the pressure drop of the endotracheal tube is eliminated and a more accurate measurement of intratracheal pressure is obtained.
In principle, it should be possible to measure the pressure at the distal end of the endotracheal tube, i.e. the intratracheal pressure, either directly by means of a sensor located at the distal end of the tube, or remotely via a pneumatic port at the distal end coupled to a remote sensor at the proximal end. However, the presence and movement of continuous secretions, such as mucus, of different and varying consistencies around the distal end of the endotracheal tube have heretofore made the, otherwise preferable, measurement of pressure at this intratracheal site difficult, particularly on a long term basis. Thus, while pressure sensors exist that are capable of measuring airway pressures on the order of 0-30 cmH.sub.2 O, the sensing element of such sensors needs to be covered by a protective, flexible cap. When the cap becomes covered by thick mucus or the like, the sensor may not be able to accurately indicate the magnitude of the intratracheal pressure. The same problem occurs with the use of a pneumatic channel or port which, when blocked by mucus or the like, is unable to transmit pressure conditions to a remote pressure sensor.
Because of the foregoing problems, techniques have been described to measure the pressure in the trachea indirectly using pressure measurements obtained from the inflated cuff of the endotracheal tube. See "Evaluation in Animals of a System to Estimate Tracheal Pressure from the Endotracheal Tube Cuff" by Wilder, Orr, and Westenskow in Journal of Clinical Monitoring 12:11-16, 1996.